As a technique related to neural network learning devices according to the related art, Japanese Unexamined Patent Publication No. H9-185595 discloses a sandglass-type neural network extension learning method. The learning method disclosed in Japanese Unexamined Patent Publication No. H9-185595 extends an intermediate layer of a sandglass-type neural network in stages and obtains a sandglass-type neural network including an intermediate layer with the minimum dimensions.